Terminology and Introduction
As used herein, the term "blank" refers to a lens element at any stage in the manufacture of the lens up to and including its finishing to a specified optical prescription but prior to edging to fit a particular spectacle frame. A "major blank" is the basic element of a composite glass lens, the major portion of the surface of which defines the basic optical property of the lens such as the distance correction. A "minor blank" is a minor component of a composite glass lens which is combined with a major blank to produce a desired optical effect. The term "blank" also refers to a mold element for casting a resin lens at any stage in the manufacture of a mold useful to cast a surface of a semi-finished or finished resin lens.
A "segment" is a portion of a glass or resin lens (or of a resin lens mold) which has an optical curvature or other optical or optically related characteristic different from remaining or other portions of the lens (or mold), such as the portion of a bifocal lens which defines a reading prescription as opposed to a distance prescription. An "addition" is a localized portion of a glass or resin lens which has an optical curvature or other optical characteristic different from remaining or other portions of the lens; a "reading addition" usually refers to the close or reading prescription portion of a multifocal lens. The terms "segment" and "addition" are often used interchangeably in describing lenses.
A lens has "positive" or "plus" curvature when it is of convex overall curvature, and has "minus" or "negative" curvature when it is concavely curved overall.
A "diopter" is a measure of the light focusing properties of a lens; a lens has a power of 1 diopter when it focuses a ray of incident light from a source of infinite distance from the lens at a point 1 meter behind the lens; a 2 diopter lens focuses an incident infinite-source ray 1/2 meter behind the lens. "Prism diopter" is a measure of the prism or light deviating effect of a lens. One prism diopter corresponds to the deviation of an emergent ray by 1 centimeter from the lens axis at a distance of 1 meter from the lens; that portion of a 1 diopter lens which lies 10 millimeters from the lens' optical axis produces a prismatic effect of 1 prism diopter.
It will be understood that any optical lens is designed to produce a particular optical effect when mounted in a particular manner to act upon light incident on the lens in a particular way. Virtually every optical lens is designed with these usage conditions in mind, regardless of whether the lens is for a camera, a telescope or an automobile headlamp. This is especially the case with ophthalmic lenses which are designed to be worn in a specified position by a particular person suffering from a specified optical difficulty to produce for that person a desired optical effect. Therefore, especially in the case of ophthalmia to which this invention pertains, it is not very helpful to consider a lens in the abstract; it is more helpful and useful to consider the lens in the context of its intended use. Similarly, an appreciation of molds for casting resin ophthalmic lenses, to which this invention particularly pertains, is aided by keeping in mind the lens produced from a given mold and the effect of that lens on a person when properly fitted to the person.
In the context of this invention, the "optical axis" of an ophthalmic lens is the line along which the user of the lens looks when the lens is properly fitted to the user and the user is looking straight ahead. The optical axis of a resin ophthalmic lens will usually have a counterpart in the mold from which the lens is cast.
General Background
Diplopia is double vision of a single object and is usually considered as a disorder of sight. Most commonly diplopia is due to imbalance between or lack of coordination of the muscles which determine the positioning of the eye for line of sight and of the muscles associated with the lens of the eye for distance focusing. Treatment of diplopia often involves the use of prismatic effects in ophthalmic lenses. Desirable prismatic effects in an ophthalmic lens can be used to train eye muscles to work together to cure or reduce the problems of diplopia. Undesired prismatic effects can induce diplopia.
As noted below, while the lens produced by use of the molds of the present invention may be used as a specific in the treatment of diplopia, it is believed that these lenses are of more significant utility in applications where the objective is to prevent induced diplopia in lenses defined for treatment of disorders other than diplopia. Specifically, a presently preferred utility of this invention, but certainly not the only utility, is in the manufacture of resin lenses of high positive spherical or aspherical curvature such as highly aspheric lenses of the type prescribed for aphakic patients. Aphakia is the absence of the crystalline lens in the human eye, and most commonly results from the removal of the natural lens from sufferers of cataracts.
The problem of undesired prismatic effects can be illustrated in the case of ophthalmic lenses for a bilateral aphakic patient, i.e, a patient who has had a cataract operation in each eye. Such lenses commonly are of highly positive diopteric power. Assume that both lenses are identical. When the user looks straight ahead, he looks down the optical axes of the lenses and no prismatic effects are encountered. If the user should look to either side, say to the left, without turning his head to align the lenses' optical axes with his lines of sight, then the lines of sight of his left and right eyes pass through highly prismatic portions of his corrective lenses, but no disturbing prismatic effects are sensed. This is so because the left eye is looking through a base-in prism and the right eye is looking through a similar base-out prism. The prismatic effects presented to the left and right eyes are equal in magnitude but opposite in sign, and thus cancel each other and present no significant prismatic imbalance.
(A base-in prism is defined as a prism or optical wedge which has its thick or base end located toward a patient's nose; a base-out prism has its base located away from the nose.)
An entirely different situation is encountered when the sight lines of the eyes of a bilateral aphakic patient converge, as when such a person looks at something close, as in reading. In such an instance, the left and right lines of sight each pass through portions of the corrective lenses which are equivalent to base-out prisms, and the effective prismatic effect on the patient is equal to the sum of the prismatic powers of the lenses. The result is a significant prismatic imbalance which, if encountered for any extended time, results in induced diplopia or double vision.
Because the corrective lenses prescribed for aphakic patients, whether suffering from monocular or binocular aphakia, inherently have high prismatic power when used off the optical axis, it is common for aphakics to read with monocular vision. This monocular vision results when, after a period of reading, the eye muscles can no longer accommodate the prismatic imbalance and diplopia is induced. The image from one eye is subconsciously suppressed in favor of the image from the dominant eye. Most persons normally tend to prefer one eye over the other. Thus, most people tend to suppress the image from the nondominant eye under all conditions of induced diplopia, although some alternate the image suppression equally between the eyes. Repeated suppression of the image from an eye in response to induced diplopia can cause atrophy in that eye and its muscles.
It is well known that the human brain and eye musculature can accommodate a base-in optical prismatic imbalance much more effectively than they can accommodate an equal amount of base-out prismatic imbalance.
Thus, for the benefit of aphakic patients as well as persons suffering from other vision disorders, a need exists for ophthalmic lenses which inherently correct for prismatic imbalance.
The Prior Art--Glass Lenses
The problem of prismatic imbalance in glass ophthalmic lenses has been solved by using, in a single lens, glasses having different indices of refraction. In a prism-compensated glass lens the major lens blank is defined of a glass having one index of refraction to which is fused, prior to finish grinding of the blank and usually in the area of the blank which will define the reading area of the finished lens, a minor blank (segment) of a glass having a different index of refraction. The segment may be placed in a recess formed in the front surface of the major blank. After the major blank and segment have been fused or otherwise adhered together, the blank is ground and polished to the desired frontal spherical or aspherical curvature to define the optically finished front surface of the lens. In such a glass lens, the frontal optical surface is usually of continuous curvature from the host glass of the major blank across the surface of the added glass (fused segment) of different refractive index. On casual inspection such a glass lens appears to be made from a single piece of glass.
Prism correction in the reading areas (additions) of glass lenses, utilizing the technique of different indices of refraction, is limited by the extra thickness created by the necessary net refractive index ratio which can be achieved between compatible glasses. With glass, for every prism diopter of effect desired in the reading addition, it is necessary to physically incorporate in the overall lens from 3 to 8 prism diopters, depending upon the ratio of the indices of refraction of the two different glasses; this significantly increases the thickness of the overall finished glass lens. This means that a reading segment in a prism-corrected glass lens is very thick. Glass is very heavy per unit volume when compared to the resins used in resin ophthalmic lenses. Corrective lenses for aphakic patients are inherently thick centrally and thus are heavy. Resin lenses are preferred for aphakic patients because they are much lighter than comparable glass lenses. Heretofore, however, prism-compensated resin lenses have not been provided for aphakics or otherwise, although a need for such resin lenses has existed for some time. Prism-compensated resin lenses have not heretofore been provided because, until now, it has not been known how to manufacture the molds from which such lenses may be cast.
In the preparation of this patent application, the following U.S. patents were considered: U.S. Pat. Nos. 2,410,145, 2,755,706, 2,890,551, 2,958,162, 3,297,422, 3,460,928, 3,649,236, 3,801,336, 3,915,723, and 3,946,982.